The invention relates to a method of transforming binary coded data for use with exchanging a serial data stream between a transmitting and a receiving facsimile transceiver and to a circuit arrangement performing such method. More specifically, the invention relates to a method of interfacing cross-connected facsimile transceivers having incompatible characteristics of horizontal resolution.
Known facsimile equipment is designed as transceiver terminals having a transmitter section which scans an original document line by line. During such line scanning operation the document is illuminated dot by dot by a light source. The light source, an optical system and a sensor are arranged on a carriage moving to and fro, while the document is transported in vertical direction after every forward and backward movement of the carriage. The electro-optical sensor converts the reflected light into binary encoded signal elements which correspond to respective black and white image elements on the document. The scanning method results in a serial data stream of such binary signal elements transmitted across a transmitting channel, usually a temporarily established channel of a telecommunication network, to a cross-connected receiving facsimile transceiver. This transceiver includes a recording section where, in turn, the data received in series control a printing operation accordingly performed line by line in order to reproduce a copy of the original document.
In accordance with the two-dimensional characteristic of the original and the reproduced copies the scanning and the recording processes have two degrees of freedom, i.e. specific horizontal and vertical resolutions. The vertical resolution is determined by the number of scanning lines per inch and the horizontal resolution is specified by the number of scanning elements and recording elements, respectively contained in a line. In other words, during the scanning operation each line is separated into a series of binary signal elements having two possible states each corresponding to a black image element and a white image element, respectively. Apparently, a cross-connected pair of facsimile receivers exchanging data have to operate in synchronism with respect to these two degrees of freedom in order to be capable of reproducing a true copy of the scanned original document.
In conjunction with recent improvements and increased possibilities of utilizing existing public networks for transmitting digital data information concerning graphical data facsimile transceivers have become more standardized and limitations in usage resulting from incompatible vertical resolution have been overcome to a reasonable degree. Other improvements, e.g. with respect to electro-optical sensors for use with scanning units of facsimile transceivers have made it possible to increase the horizontal resolution previously employed with the restraint of the loss of compatibility with older equipment.
It is conceivable to overcome this limitation by utilizing a method of aligning the operation of facsimile transceivers with different horizontal resolution. Such alignment could be achieved by interrogating the binary signal elements furnished by the transmitting transceiver with a first frequency as predetermined by the scanning operation by means of an evaluating signal of a second frequency which is predetermined by the recording operation of the receiving facsimile transceiver. The resulting output signal varying with the second frequency represents binary signals having a first state if a rising edge of the evaluating signal coincides with the corresponding first state of the binary signal contained in the data stream received from the transmitting transceiver. Correspondingly, the second states of the output signal are derived. The second frequency may be higher or lower than the first frequency in dependence upon the ratio of the horizontal resolutions of the receiving transceiver and the transmitting transceiver.
Such a conceivable method of aligning two facsimile transceivers having different horizontal resolutions has the disadvantage that rapid changes of the binary values in the serial data stream may not be exactly reproduced. Isolated changes of status may be slipped if the second frequency is lower than the first frequency, or the incidental phase difference between the first and the second frequency can suppress a discrimination of the change of status of the original binary signal. In other words, such evaluating measure reduces the information contents of the original serial data stream wherein the information contents are determined by the number and type of signal changes between the two possible states of subsequent binary signal elements.